The invention relates generally to an alarm system, and more particularly to an apparatus which may be used to monitor the magnitude of a varying physical quantity such as the running speed of an automobile to produce an alarm when the speed exceeds a preset threshold value.
As is well recognized, most of automobile accidents are caused by a car driving at an improper running speed. While legal constraints are imposed on the car drivers as by traffic signs which indicate the speed limitations, the observation of the traffic rules is left to the discretion of the car drivers, and the speed limitation is frequently ignored. Though traffic control has been exercised to supervise the running speed of automobiles in a given interval or at a given point of the road so that any violation of the speed limitation may be punished. However, it is difficult to expect the psychological effect upon the car drivers of such traffic control throughout the entire car traffic way. On the other hand, there has been a proposal to incorporate a device on the automobile which is operative to produce an alarm when the running speed exceeds a preset threshold value, and several versions of such apparatus have been manufactured. In one construction, a sensor is mounted at a given position on the speedmeter of a car which corresponds to a given value, so that the detection of the pointer by the sensor activates a buzzer or indicator lamp. However, the known alarm apparatus operates on a single threshold value, which cannot be selectively changed in accordance with the different speed limitations on the individual roads.